Causes of Knife Corrosion in 304 Stainless Steel

Infiltrating stabilized alloying elements such as Da, Ju, and Jun into the weld metal of 304 stainless steel can remove some forms of intergranular corrosion, but it is also prone to knife-like corrosion, which appears along the fusion line of the weld metal Corrosion phenomenon similar to knife cutting. This kind of corrosion will spread deeply to the stainless steel material along the weld fusion line, which will seriously affect the service life of the welded structure.

304 stainless steel plate

The scene and conditions of knife corrosion are not exactly the same as intergranular corrosion. Intergranular corrosion will only form in the weld seam and its surrounding areas. Stainless steel materials that have not been welded will not produce knife-like corrosion if they are heated for a long time in the dangerous temperature range. When 18-8 stainless steel contains stabilizing elements Chin, the quality of carbon The fraction is lower than 0.06%, and there will be no knife-like corrosion: in 304 stainless steel with the same carbon content, the weld with stabilizing elements may produce knife-like corrosion, while the weld without stabilizing elements will not. Knife-like corrosion will occur; stabilized 304 stainless steel containing silver or silver may produce knife-like corrosion, while this type of steel in a solution-treated state may produce intergranular corrosion under the premise of intergranular corrosion. Knife corrosion.

Knife-like corrosion will occur on the outside of the weld fusion line, but intergranular corrosion will not occur in the sensitization temperature range of the heat-affected zone. It can be determined that the welded 304 stainless steel contains stabilizing elements and is subject to stabilization treatment. of. As mentioned above, the 304 stainless steel with simple solution treatment will first have the tendency of intergranular corrosion in the sensitization temperature range due to the influence of welding heat cycle.

This kind of stainless steel in the stabilized treatment state produces knife-like corrosion, which is usually caused by reheating after welding. It is affected by the heat treatment process after welding or the stay of the welded components in the sensitization temperature zone. It is the subsequent weld bead or the thermal influence of the weld seam.

If the structure of the single-pass weld of 304 stainless steel after welding is heated by the sensitization temperature, it will also have a tendency to intergranular corrosion in the overheated section of the heat-affected zone. Because the overheating section of the previous weld is relatively narrow, the area where it is reheated and sensitized is also narrow. Therefore, under the action of the corrosive medium, the knife-edge incision, that is, knife-like corrosion, is mainly formed in the overheating zone of the welded joint.

How to identify 304 stainless steel

Today, when 304 stainless steel is widely used, people often think that "stainless steel" means "stainless steel". If it is rusty, it must be a counterfeit and inferior product. "It is not absolutely rust-free, but relatively rust-free under certain conditions. It is difficult to distinguish stainless steel 304 and 202 from the appearance and magnetism on the market. How can we easily distinguish them? Here are some simple identification methods:

Stainless steel identification method

1. Friction spark identification method

Use 304 material and 202 material respectively, and put them on a high-speed rotating grinder to see the sparks generated. Due to the high temperature of friction, the number and shape of sparks generated when various element particles are oxidized are different to identify the chemical composition and approximate content of the material.

2. Identification method of stainless steel potion

You can go to stainless steel accessories stores (such as Jiangtou Fansheng, Xin Yongsheng) to buy, the unit price is 15 yuan a bottle. Put the stainless steel identification potion on different materials for two to three minutes to distinguish them, and different colors will be displayed, and then compare them with reference to the manual.

3. Surface observation

First of all, check whether there is rust on the surface. If there is no rust, it can be determined that it is stainless steel, but it cannot be judged by this.

4. Potion detection

There is material testing water on the market. After dripping on it, different colors correspond to different materials. It is very simple and convenient, and you can roughly judge which type of stainless steel material it is.

5. Grinding inspection

Grinding wheel to see the state of sparks, this method needs to be identified by experienced people.

6. Spectral detection

Instrument identification is the safest and most accurate method. First remove a layer of the surface with a grinder, but the surface must be smooth and then use a spectrum to read the metal elements in the material in detail, and then compare the material. This method is the most reliable and authoritative.

Although the above stainless steel identification methods can quickly identify stainless steel 304 and 202 to a certain extent, there will be certain errors, and if you want to accurately distinguish the specific type and grade, you still have to send it to a special organization for professional testing. 100% sure.